The invention relates to a device for recording and/or reproducing information on/from a rotary information carrier, including
means for rotating the information carrier,
conversion means for the conversion of a write signal into a detectable pattern on the information carrier, or for generating a read signal in response to a detectable pattern on the information carrier,
control means for supplying at least one control signal to the conversion means in response to an error signal,
detection means for the generation of a detection signal which is a measure of a periodic variation in the error signal.
The invention further relates to a method of recording and/or reproducing information on/from a rotary information carrier, comprising the steps of
rotating the information carrier,
converting a write signal into a detectable pattern on the information carrier, or generating a read signal in response to a detectable pattern on the information carrier,
generating an error signal which is related to a parameter which influences the conversion,
generating a control signal for optimizing the parameter in response to the error signal,
generating a detection signal which is a measure of a periodic variation in the error signal.
A rotary information carrier is, for example, a disc-shaped optical information carrier such as a CD, a DVD or a DVR, a magnetic information carrier or a magneto-optical information carrier.
Rotary information carriers may exhibit various errors which complicate reading of and writing on such information carriers. In the case of an error of a first type (skew), shown diagrammatically in FIG. 1A, the normal to the surface of the information carrier 1 is not parallel to its axis of rotation 2 but deviates therefrom by an angle xcex1. During rotation of the information carrier the surface thereof moves up and down with an amplitude h. This means that a continual adjustment of an axial setting of the conversion means is necessary, for example an axial setting of a focusing element in an optical read/write head, in order to follow this periodic excursion.
An error of a second type, shown diagrammatically in FIG. 1B, is eccentricity. In this case, the axis of rotation 2 of the information carrier 1 does not coincide with a point 2xe2x80x2 about which the tracks of the information carrier are centered. In this case the conversion means should perform a periodic radial movement of an amplitude r in order to enable a track to be followed.
When the information carrier exhibits an error of a third type, shown diagrammatically in FIG. 1C, the mass of the information carrier is not distributed symmetrically about the axis of rotation 2. In this case a rotation of the information carrier 1 results in the information carrier surface being moved periodically in a radial direction with an amplitude r as well as in a vertical direction with an amplitude h. This requires a continual adjustment of the conversion means in these two directions.
EP 821 356 A2 describes a method of operating a device for reading CD-ROM information carriers. Said device detects whether the disc placed on the device exhibits an eccentricity or unbalance. This detection is effected on the basis of the radial error signal or a control signal for the motor for the rotation of the information carrier. If this detection shows that the information carrier exhibits an eccentricity or unbalance the speed of rotation of the information carrier is reduced. However, a drawback of this method is that a reduction of the speed of rotation also results in a reduction of the speed of reading and/or writing.
It is an object of the invention to provide a device of the type defined in the opening paragraph, which offers an alternative to the reduction of the speed of rotation.
It is a further object of the invention to provide a method of the type defined in the second paragraph, which offers an alternative to the reduction of the speed of rotation.
To this end, according to the invention, the device of the type defined in the opening paragraph is characterized by means for influencing the response of the control means to the error signal in dependence on the detection signal.
According to the invention the method of the type defined in the second paragraph is characterized by the response to the error signal being influenced in dependence on the detection signal.
By influencing the response (of the control means) to the error signal it is possible to raise the reliability with which the conversion means read a signal from the information carrier or write said signal to the information carrier. The invention is of particular interest for a device for reading of and/or writing onto optical information carriers. In such a device the conversion means usually take the form of a read/write head including a radiation source, means form imaging radiation generated by the radiation source so as to form a scanning spot on the information carrier, and means for imaging radiation reflected from the information carrier onto detection means. Such a read/write head usually has a first actuator for the axial tracking of the information carrier and a second actuator for the radial tracking of the information carrier.
An embodiment of the device in accordance with the invention is characterized in that the error signal is a radial error signal. With the aid of this error signal it is possible to determine whether the information carrier exhibits an eccentricity.
A further embodiment of the device in accordance with the invention is characterized in that the error signal is a focus error signal. With the aid of this error signal it is possible to determine whether the information carrier exhibits skew.
The response can be influenced in different manners, depending on the nature of the error of the information carrier.
An embodiment is characterized in that the adjustment means influence a bandwidth of the control means. An increase of the bandwidth is particularly favorable in the case of an information carrier which exhibits an unbalance. In this case, the radial and the axial error exhibit not only a disturbance at a fundamental frequency which corresponds to the frequency of rotation of the information carrier but also disturbances at higher harmonics of this frequency.
A further embodiment is characterized in that the control means include a PID controller and the adjustment means influence one or more gain factors of the PID controller. In the case of an eccentric information carrier it is favorable, for example, to increase the integrator frequency.
An embodiment of the device in accordance with the invention is characterized in that the device further includes detection means for the generation of a further detection signal which is a measure of the influence of the angular velocity with which the information carrier is rotated on the detection signal, the device having the adjustment means also in order to influence the response of the control means to the error signal in dependence on the further detection signal. The influence of the speed of rotation on the detection signal is an indication of the degree of unbalance of the information carrier. The influence of the speed of rotation on the detection signal becomes greater as the unbalance of the information carrier is greater.
In an embodiment of the device in accordance with the invention the detection means include a filter and a peak detector. The filter transmits the disturbance components which are relevant to the information carrier error to be detected. The peak detector forms a practical method of deriving the detection signal from the filtered signal. Alternatively, it is possible to use a detector which detects the means square value of the signal.
Preferably, the filter is a digital band-pass filter whose sample frequency is locked to the angular velocity of the information carrier. The transmission of the error signal component to be detected is then substantially independent of the angular velocity at which the measurement is effected.
It is not adequate in all cases to influence the response of the control means to the error signal in dependence on the detection signal. A favorable embodiment of the device in accordance with the invention is characterized in that, as an additional measure, the adjustment means further influence the speed of rotation of the information carrier in response to the detection signal. In this embodiment the speed of rotation is reduced, for example, if the detection signal, or the further detection signal derived therefrom, exceeds a predetermined value.